1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the same, and more particularly, it relates to a semiconductor device having a semiconductor element layer formed on a substrate and a method of fabricating the same.
2. Description of the Background Art
In general, a light-emitting diode device or a semiconductor laser device is known as a semiconductor device having a semiconductor element layer formed on a substrate. Such a semiconductor device is disclosed in Japanese Patent Laying-Open No. 11-214798 (1999), for example.
The aforementioned Japanese Patent Laying-Open No. 11-214798 discloses a nitride-based semiconductor laser device having a plurality of nitride-based semiconductor layers formed on a nitride-based semiconductor substrate. More specifically, an n-type nitride-based semiconductor layer, an emission layer consisting of a nitride-based semiconductor and a p-type nitride-based semiconductor layer are successively formed on an n-type GaN substrate in the nitride-based semiconductor laser device disclosed in the aforementioned Japanese Patent Laying-Open No. 11-214798. A ridge portion serving as a current path portion is formed on the p-type nitride-based semiconductor layer, while a p-side electrode is formed on the ridge portion. An n-side electrode is formed on the back surface of the n-type GaN substrate.
When dislocations are present on the back surface of the substrate in the aforementioned semiconductor device having an electrode on the back surface of the substrate, current flows to regions of the back surface of the substrate having the dislocations, to result in development of leakage current. In the aforementioned Japanese Patent Laying-Open No. 11-214798, therefore, the n-type GaN substrate is prepared by lateral growth thereby reducing the number of dislocations present in the n-type GaN substrate. More specifically, a mask layer is formed on a prescribed portion of a sapphire substrate, and an n-type GaN layer is thereafter laterally grown on the sapphire substrate through the mask layer serving as a selective growth mask. At this time, the n-type GaN layer is selectively longitudinally grown on portions of the sapphire substrate formed with no mask layer, and thereafter gradually grown in the lateral direction. The n-type GaN layer is laterally grown to laterally bend dislocations, thereby inhibiting the dislocations from longitudinal propagation. Thus, the n-type GaN layer is so formed as to reduce the number of dislocations reaching the upper surface thereof. Thereafter regions (the sapphire substrate etc.) including the mask layer located under the n-type GaN layer are removed thereby forming an n-type GaN substrate having a reduced number of dislocations.
In the method of the aforementioned literature, however, regions having concentrated dislocations are disadvantageously formed on the portions, allowing longitudinal growth of the n-type GaN layer, formed with no mask layer. If the n-side electrode is formed on regions of the back surface of the n-type GaN substrate having concentrated dislocations when the n-type GaN substrate is prepared from the n-type GaN layer including the regions having concentrated dislocations, current flows to the regions of the back surface of the n-type GaN substrate having concentrated dislocations to disadvantageously result in development of leakage current. In this case, optical output is unstabilized when the device is subjected to constant current driving, and hence it is disadvantageously difficult to stabilize operations of the device.